1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus and an ultrasonic diagnostic method using nonlinear signals from a living body or ultrasonic contrast medium.
2. Description of the Related Art
An ultrasonic apparatus can noninvasively obtain a tomographic image of soft tissues inside a living body from the surface of the body using reflection of an ultrasonic wave and has advantages such as compactness, low price, capability of real time imaging, increased safety due to a lack of X-ray dosing, capability of blood flow imaging, etc., when compared to other diagnostic devices, such as X-ray diagnostic apparatus, X-ray CT apparatus, MRI apparatus or nuclear medicine diagnostic apparatus, and the like. Because of such advantages, ultrasound is currently widely used in many medical fields such as cardiac medicine, abdominal medicines such as gastroenterology, urology, obstetrics, gynecology, as well as others.
There are a variety of imaging methods for the ultrasonic diagnostic apparatus. “Contrast Echo” method is one of the important techniques to obtain an ultrasonic image in which scattered echo signals are enhanced by using an ultrasonic contrast medium containing microbubbles injected into a blood vessel of an object.
Recently, new contrast media for intravenous injection have emerged and imaging methods suitable for these contrast media have also been developed. For example, such methods include a Filter method (an imaging method using one pulse per scanning line: U.S. Pat. No. 5,678,553), a Doppler method (an imaging method using more than two in-phase pulses per scanning line), a Phase Inversion method (an imaging method using two 1800 out-of-phase pulses per scanning line: U.S. Pat. No. 5,632,277), a Phase Inversion Doppler method (an imaging method using more than three pulses alternated by 180° from pulse to pulse per scanning line: U.S. Pat. No. 6,095,980), the contents of which are herein incorporated by reference.
When performing a Contrast Echo method with any of the above methods, conventionally ultrasonic waves with medium or high acoustic pressure, such as MI (mechanical index: a value obtained by normalizing peak of negative sound pressure by a reference sound pressure of 1 Mpa) value of more than 0.5 have been used. This is typically done to obtain a prominent contrast enhancement effect by collapsing the microbubbles in the contrast medium. For example, when using one of widely used contrast medium, Levovist, manufactured by Schering, the image may not be properly enhanced unless ultrasonic waves with high acoustic pressure, such as those having an MI value of more than 0.8 are used.
Transmission of ultrasonic waves with high acoustic pressure and the subsequent collapse of microbubbles greatly affects ultrasonic imaging. When an ultrasonic wave with high acoustic pressure is propagated through tissue, a harmonic component is generated in an echo. However, the above-mentioned Filter method or Phase Inversion method can not separate a harmonic component from tissues (hereinafter “THI component”) from a harmonic component from bubbles. Accordingly an image of bubbles obtained with the Filter method or the Phase Inversion method may not have enough contrast, and it may be difficult to distinguish between blood flow and a parenchymal contrast-enhancement in a contrast echo image.
Further, a collapse of bubbles creates a broadband Doppler signal when more than two ultrasonic beams are transmitted per scanning line. This broadband Doppler signal, called a pseudo-Doppler signal, can be utilized to produce an image because tissue and THI components in the fundamental signal can be suppressed by processing high-pass filter to suppress a signal whose motion is slow.
However, a color Doppler image based on the pseudo-Doppler signals would result in an image of thin blood vessels and contrast-enhancement in parenchyma with many aliasing points, which does not indicate proper blood flow velocities. Because the pseudo-Doppler signals unlike normal Doppler signals from blood flow do not indicate proper blood flow directions. Therefore, in most cases when an ultrasonic image obtained with contrast echo method is displayed, power Doppler is used instead of color Doppler, which is generally suitable for showing blood flow velocity.
Accordingly, in order to solve these problems, a system capable of performing good color Doppler imaging using an ultrasonic contrast medium, which achieves enhancement even if a MI value is 0.1 or lower, is proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2003-102726). According to the system, on condition that the MI is low and the occurrence of THI components is suppressed, harmonic signals reflected from the contrast medium are extracted and a power signal and a velocity signal from the contrast medium are calculated. On the basis of the power signal, the velocity signal, and B-mode information of fundamental wave, only a B-mode image is displayed in grayscale before the contrast medium is injected. When blood flow in vessels is enhanced by the contrast medium, the direction of blood flow is displayed in red or blue. When blood flow in parenchyma is enhanced by the contrast medium, blood flow is displayed in green.
The system uses a very low ultrasonic power at which the MI value is 0.1 or lower. In addition, since second higher harmonic wave is used, imaging is seriously affected by attenuation dependent on frequency. Disadvantageously, therefore, the S/N ratio may be insufficient, resulting in poor penetration.
In principle, signals obtained by phase inversion or phase inversion Doppler are even-order higher harmonic wave. Accordingly, as a nonlinear signal capable of being used on condition that bubbles are not disrupted in filtering, phase inversion, and phase inversion Doppler, second higher harmonic wave are used practically. Regarding available higher harmonic wave other than the second higher harmonic wave, principally, third higher harmonic wave are obtained by filtering. Disadvantageously, a probe for a very wide band of frequencies is required and imaging with the third higher harmonic wave is seriously affected by attenuation dependent on frequency. Therefore, the use of the third higher harmonic wave is not adequate for the purpose of increasing penetration.
Generally, according to techniques using second higher harmonic wave, sensitivity is low. As solutions to harmonic problems, e.g., approaches using nonlinear signals of fundamental wave area are proposed. According to one of the approaches, a transmission pulse is transmitted two times such that the amplitude of the first pulse is different from that of the second and received signals are subjected to gain compensation, thus obtaining the different there between (see, for example, U.S. Pat. No. 5,577,505). According to another approach, a transmission pulse is transmitted two times such that the amplitude and the phase of the first pulse are different from those of the second (see, for example, U.S. Pat. No. 6,063,033). According to still another approach, second higher harmonic wave is obtained with high sensitivity by a pulse compression technique using a chirp signal (see, for example, U.S. Pat. No. 6,213,947).
In the above conventional solutions to the degradation in sensitivity caused by the use of second higher harmonic wave, i.e., the approach using nonlinear signals of fundamental wave area, the approach for changing both the amplitude and the phase of transmission pulse, and the approach for obtaining second higher harmonic wave with high sensitivity based on the pulse compression technique using a chirp signal, amplitude information such as B-mode information is merely used for imaging. In contrast echo imaging, therefore, velocity information of blood flow cannot be extracted from nonlinear signals of fundamental wave area with accuracy.
Further, not only in contrast echo imaging, it is desired to obtain nonlinear signals from living body with high sensitivity and high penetration.